How is the contravention of the Jews achieved on their days of veneration: is it by fasting, so as not to celebrate, or by breaking the fast and not intending to fast, taking into account the Hadith of Umm Salamah: "The Messenger of Allah, peace and blessings be upon him, used to fast on Saturday and Sunday more than he would fast on other days, and he would say: 'These are the days of 'Eid for the idolaters, and I love to contravene them,' and the scholars' reasoning for the Prophet's prohibition of fasting on Saturday?"

It is known from the religion that it is forbidden to follow disbelievers in what distinguishes them, and to oppose them by refraining from celebrating their holidays. The Prophet, peace and blessings be upon him, ordered to abandon celebration (of their holidays) and did not order any additional acts like fasting. Intentionally fasting on these days is an illicit act, and a number of scholars have explicitly stated its detestability (karāha), due to the veneration it entails for these days. Therefore, fasting on the days of Nowruz and Mehragan and the like is considered detestable by many scholars, because they are two days venerated by disbelievers, and singling them out for fasting constitutes agreement with them in their veneration. As for the hadith of Umm Salamah that the Prophet, peace and blessings be upon him, used to fast on Saturday and Sunday and would say: "They are the days of the polytheists' festivals, so I love to oppose them," there is doubt about its authenticity. Even if authentic, its meaning cannot be extended to all disbelievers' festivals. Some scholars consider it detestable to fast on the disbelievers' festivals that are not based on the Arab calendar, and recommend fasting on those that are based on the Arab calendar, in order to reconcile the various narrations.